Interior permanent magnet (IPM) machines are favored for fuel cell and hybrid electric vehicle operations due to their desirable characteristics—good torque density, high overall efficiency, and relatively constant power range, etc. The rotor field in a permanent magnet machine is obtained by virtue of its structure, unlike other machines such as induction, switched or synchronous reluctance machines, in which the field is generated by a stator current supplied by a source. As a result, permanent magnet machines exhibit superior efficiency as compared to other such machines.
An IPM machine typically includes one or more rotor barriers, which include permanent magnets and/or air gaps (which may be filled with a nonmagnetic material). These rotor layers act as barriers to the permanent magnet field of the lower primary magnet layer, reducing the air-gap magnet flux, and lowering the machine back EMF and losses induced by the permanent magnet field.
Traditional IPM rotors are unsatisfactory in a number of respects. For example, it is desirable to use less expensive ferrite magnets in place of rare earth magnets (such as NdFeB) to reduce cost. However, such ferrite magnets are often at risk for demagnetization under certain operating conditions (e.g., low temperature and high d-axis current).
Accordingly, it is desirable to provide improved, low-cost IPM rotor designs with reduced susceptibility to demagnetization. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.